This invention related to an improved mechanism for obtaining human fingerprints without the use of ink by means of optical scanning.
It is known to provide an optical scanning mechanism utilizing a prism against which the finger is placed. By using the principle of Total Internal Reflection it is possible to obtain a view through such a prism of the areas of contact of the ridges of the human fingerprint with the surface of the prism, due to the frustration of the mechanism of Total Internal Reflection at those points where the skin ridges make contact with the surface. When so viewed, there is a difference in contrast between the points of contact of the fingerprint and the surrounding area. See for example, U.S. Pat. No. 3,482,498 (Becker), U.S. Pat. No. 4,414,684 (Blonder), U.S. Pat. No. 3,200,701 (White), U.S. Pat. No. 3,968,476 (McMahon), U.S. Pat. No. 4,210,899 (Swonger).
U.S. Pat. No. 3,527,535 (Monroe) illustrates the principle of using a dark field background to enhance the contrast of such fingerprints obtained using frustrated Total Internal Reflection. Monroe utilizes a dark opaque background in order to produce a light on dark image, significantly improving the contrast ratio available in the image. However, illumination is provided through the sides of the prism only, reducing the amount of light available through scattering from the points of contact of the skin ridges with the surface of the prism.